The alteration of (Ca(2+)-Mg2+)-ATPase activity in the plasma membranes of regenerating rat liver after a partial hepatectomy was investigated. Liver was surgically removed about two thirds of that of sham-operated rats. The reduced liver weight by partial hepatectomy was restored about 50% at 24 h after the surgery, and it was completely restored at 72 h. Regenerating liver significantly increased calcium content and plasma membrane (Ca(2+)-Mg2+)-ATPase activity between 12-48 h after hepatectomy. Those increases were maximum at 24 h after the surgery. The regenerating liver-induced increase in hepatic plasma membrane (Ca(2+)-Mg2+)-ATPase activity was completely abolished by the presence of anti-regucalcin IgG (1.0-4.0 micrograms/ml). The regenerating liver-induced increase in hepatic plasma membrane (Ca(2+)-Mg2+)-ATPase activity was clearly inhibited by N-ethylmaleimide (2.5 and 5.0 mM) addition into the enzyme reaction mixture. This NEM effect was also seen for the activatory effect with regucalcin (0.25 microM) addition on the enzyme activity in the plasma membranes from normal rat liver. The endogenous regucalcin may play a cell physiological role in the activation of the plasma membrane (Ca(2+)-Mg2+)-ATPase to maintain the intracellular calcium level in regenerating rat liver.